Hi,
According to Nitrokey 3 FAQ - Nitrokey Documentation the NK3A Mini should allow up to 100 passkeys and the NK3C up to 35.
However, nitropy fido2 list-credential shows ~61 (5 used, estimated 56 slots left) and ~11 respectively (4 used, estimated 7 slots left).
The keys are currently not used for anything else (there are no GPG / PIV keys or secrets stored).
Firmware version is 1.8.3 on both.